Several prior art sound reproducing systems have included the loudspeaker in a feedback for reducing loudpeaker distortion, and for allowing use of smaller loudspeakers and smaller loudspeaker enclosures. Such prior art systems, especially those which include means for magnetically sensing the axial motion of the associated loudspeaker cone, have considered neither the detrimental effects due to electrical interference from the main electromagnetic loudspeaker structure, nor the proper frequency shaping of the motional feedback signal to cause the loudspeaker to respond linearly to the input source signal. Such prior art sound reproducing systems are cited in U.S. Pat. No. 3,798,374 entitled "Sound Reproducing System Utilizing Motional Feedback", issued on 3/19/74 to Applicant herein.
Further, U.S. Pat. No. 3,821,473, entitled "Sound Reproduction With Driven and Undriven Speakers And Motional Feedback", issued on 6/28/74 to Mullins, discloses amplifier 4, device 12, and speakers 14 and 16. In such system there is included an undriven speaker and each of the speakers mounted in the enclosure have different resonant frequencies and different motional devices attached thereto. The outputs of the motional feedback devices are combined to provide a negative feedback signal to the amplifier. The system also includes motional sensor 18. Such sound reproduction system relates to combined motional feedback control of a driving and a driven speaker in a single enclosure. There does not appear to be a description of any particular type of motional feedback sensing means although acceleration sensing is mentioned.
U.S. Pat. No. 3,878,748, entitled "Oral Cavity Controlled Electronic Musical Instrument", issued on 4/22/75 to Spence, discloses sensor coil 58. FIG. 9 of such patent refers to a method of divesting a separate sensing coil of interference from the voice coil. Such arrangement appears to be a ramification of bridge type feedback control.
U.S. Pat. No. 4,025,722, entitled "Method And Apparatus For Recording", issued on 5/24/77 to Karron, discloses speaker 20 including voice coil 18 and auxiliary winding 30. The output of auxiliary winding 30 is coupled to primary winding 32 of transformer 34, but does not appear to be fed back to amplifier 16.
The North American Philips Corporation distributes a sound reproducing system including a signal source, an electronic cross-over, a comparator, a low frequency amplifier, a woofer, a piezoelectric sensor, a high frequency amplifier, a second crossover, a midrange speaker, and a tweeter speaker. In such sound reproducing system, acceleration feedback is utilized but only in the so-called woofer speaker.
However, none of the aforementioned prior art sound reproducing systems includes the particular cone motion sensing structure of the present invention to produce a motional feedback signal and utilize such motional feedback system as herein described.
Objects of the present invention are therefor to:
utilize motional feedback in a sound reproducing system for reducing loudspeaker distortion, for providing a uniform sound energy output, and for effecting linear loudspeaker response to the input source signal; PA0 utilize motional feedback in a sound reproducing system wherein relatively small loudspeakers and relatively small loudspeaker enclosures are required; PA0 utilize the stray magnetic field of the loudspeaker's main electromagnetic structure to provide a motional feedback signal readout of the loudspeaker cone; PA0 reduce acoustic coupling between the sound energy output and the cone motion sensing structure; and PA0 reduce obstruction of the sound energy output by the cone motion sensing structure. PA0 the motional feedback signal generated by the cone motion sensing structure is functionally related to axial cone velocity; PA0 the cone motion sensing structure utilizes the stray magnetic field of the main electromagnetic structure to provide such motional feedback signal; PA0 the motional feedback signal from the cone motion sensing structure is substantially free from components due to current in the loudspeaker voice coil whereby the motional feedback signal is a function of cone motion only; and PA0 the cone motion sensing structure is rearwardly recessed from the loudspeaker cone such that acoustic coupling between the sound energy output and the cone motion sensing structure is reduced and obstruction of the sound energy output by the cone motion sensing structure is eliminated to improve the natural quality of the radiated sound energy output. PA0 relatively small loudspeakers and relatively small loudspeaker enclosures can be utilized; PA0 loudspeaker diaphragm performance is substantially independent of enclosure characteristics; and PA0 a separate electromagnetic structure is not required for the functioning of the cone motion sensing structure.